Old house with sheets of new foam insulation on outside wall.

Old House Exterior Remodeling:

Wall Preparations For New Siding

In This Article: Related Articles:

Skill Level: 2-3 (Basic to Intermediate)

Time Taken: 4-8 hours per wall.

By Bruce W. Maki, Editor

This article is part of a series on siding replacement. The entire story can be found in Replacing Old Wood Siding and Trim With Low-Maintenance Materials - A Quick Tour


Once the old siding was removed we spent a few hours doing some important preparation work. The main purpose of this work is to improve the insulation value of the walls and prevent water damage if  rain should penetrate behind the siding.

Improving the insulation has two components: Increasing the R-value of the total wall assembly, and reducing the amount of air infiltration, which is commonly known as draftiness.

After the siding had been stripped the house looked completely different. Like many older houses, this house has solid wood sheathing nailed across the studs.

This house is different from most older homes because it has awnings over the windows. These added some complication to the siding replacement job.


We removed the siding all the way down to the water table trim, which is the 2-part white-painted set of boards just above the foundation. I think the original purpose of the water table was to kick the rain water outward and keep it from getting near the foundation. Or maybe it's strictly decorative.

I've seen people leave the water-table attached to the house during siding replacement, but we intend to increase the thickness of the wall (by adding foam insulation), so we removed the trim.


Many years ago, probably during the 1970's, somebody had blown-in insulation installed. This operation involves drilling holes through the siding and sheathing, then inserting a nozzle to inject expanding foam into the space between the studs.

The holes in the siding are plugged afterwards, but these plugs don't always stay put.


These holes are pretty big, about 2½ inches in diameter. We have found old bird's nests, and bird skeletons, in other wall cavities.

If a plug becomes loose it creates a perfect home for critters.

If you look in the hole, you can see the back of the lath and the plaster that oozes between the lath boards. This is supposed to be an insulated wall. Why is there so much insulation missing?

When you consider that the only material on top of the sheathing was a layer of paper and the clapboard siding, it becomes obvious why this old house was so drafty. And that blown-in formaldehyde insulation was terribly lame because it shrank after curing, leaving half-inch gaps all around it. That stuff did nothing to reduce air infiltration, I suspect it actually increased draftiness because of the holes and the ill-fitting plugs.

At this stage many builders would apply a house wrap, such as Tyvek or Typar, to reduce air infiltration and form a barrier to water droplets that get past the siding. I've read about some problems with those materials, because they can stop water droplets from going anywhere, which can cause all sorts of problems such as water damage to siding and trim.

Here's a problem:

Notice the complex millwork (red arrow) where the soffit corner meets the vertical corner trim. We would like to remove the corner trim, install ½" thick foam insulation, and then replace the corner trim. But... this curvaceous soffit board prevented easy relocation of the vertical boards.


A view of the other side. This board encloses the soffit where it meets the gable-end of the house.

We could have removed that board and cut it 1/2 inch narrower. But that amount of trouble, plus the effort in replacing the corner trim, was not worth the outcome... gaining a narrow strip of additional insulation.

By adding a layer of foam we are causing the siding and some of the trim to "stand off" the wall by a fraction of an inch... the thickness of the insulation. There are several factors that limit how thick this new insulation can be. We used ½ inch foam. I'll discuss the reasons later.


Fastening The Sheathing:

I have worked on many old houses, and I've noticed that over the years nails become loose. This isn't necessarily a problem, but it does allow more structural twisting and flexing during strong winds. This movement is called racking, and it doesn't help anything. Racking can be noticed by all the creaking sounds during windy weather.

I've also noticed a lot of badly rusted nails around the exterior parts of old houses. Sometimes these nails have corroded down to nearly nothing. That can't be good.

When I see a wall or roof that has been stripped down to the sheathing, I see a big opportunity... a chance to make repairs and improvements at minimal cost and effort. Most carpenters I've known just hammer in any loose nails and proceed with the job. I have read about studies that show that wall sheathing secured with deck screws can be up to 3 times stronger than sheathing secured with nails. My experience in construction and demolition agrees with that conclusion, so I figure... why not add a little extra holding power.

After hammering in the loose nails, we used a collated screw gun to drive 1-3/4 inch deck screws through the sheathing into the studs. This tool made short work of fastening the sheathing to the wall.


A closer view. This is actually two tools:

  • The yellow and black part is a Dewalt DW257 deck/drywall screw gun, about $90.
  • The silver part in front is a Senco DuraSpin DS200 collated screw feeder attachment, about $50.

When combined with a drywall or deck screw gun, the DS200 can drive a lot of screws in a hurry. For fifty bucks this thing is amazing, and it fits MANY popular screw guns.

Fastening old solid wood sheathing with screw gun.


Sometimes the screws didn't go all the way in, so we had to finish driving them with my impact driver. This happens because the operator momentarily stops pushing on the screw gun which causes the bit to stop spinning.

Once you get the hang of it, almost all of the screws go in with no problem.


Along the edge of the vertical corner boards there was a thick mess of old paint. When we caulk between the edge of the new siding and that vertical trim I wanted a clean wood surface for the caulk to adhere to. This is important because if we simply caulked to the old paint, and the paint later breaks away, that will create a gap for rain to get behind the siding or trim.

To scrape away this paint we used a narrow paint scraper with a triangle-shaped carbide insert. Carbide stays sharp for a long time and can tolerate hitting nails.


Water Repellent:

Once the sheathing had been fastened we could start covering the wall.

The first covering was 15# felt, also called tar paper. This is meant to act as a water barrier. Since tar paper is a moisture retarder, and not a true vapor barrier, water vapor can pass through it slowly, over time. But if any rain gets behind the siding and foam, this layer of protection should keep liquid water from getting into the walls and causing structural damage.

We installed the felt with a hammer tacker, which is a staple gun that you swing like a hammer. An ordinary staple gun works too, but it's a lot slower.

We fastened a strip of 1/2" plywood at the bottom of the wall. The bottom edge of this plywood lies exactly at the bottom edge of the water table trim.

We snapped a chalk line first, stretching from the existing water table boards at each corner. Later, the new water table trim will be aligned with this plywood, so accuracy here will affect the entire project.

Why put plywood here? I wanted a good nailing base for the new water table trim, but more important, I wanted the lower edge of the wall assembly to be completely solid. I have heard of bugs (like termites) and small animals digging through soft materials like foam insulation, so I wanted a solid line of defense facing any critter that crawls up the foundation wall looking for a warm home.


We continued installing pieces of felt. Since felt is only 3 feet wide, it's easier too handle than housewrap, which is 9 or 10 feet wide and comes in large, unwieldy rolls.

At this point there were three rows of felt installed. Each sheet of felt needs to overlap the piece below by at least 2 inches.



We installed 1/2" thick polyisocyanurate foam insulation (also called rigid foil-faced foam) over the felt. The foam was fastened with 2" cap nails, which are ring-shank nails with a plastic cap that resists tearing through the foam.

In a pinch I have used roofing nails to install foam insulation, but if a big windstorm comes along the foam may go flying.

Cutting foam insulation is easy... I just use a drywall T-square (or a long straight-edge) and a sharp knife. But handling foam in breezy weather can be tricky... the wind can easily fold a sheet in half. I just hold it by a corner and let it flop like a flag.

Here we've progressed farther up the wall. There can be a lot of ladder lifting and moving when doing siding work on a two-story house.

We used a couple of 8 foot step-ladders for the first 10 feet of height, and then progressed to extension ladders for the remainder.


Nailing sheets of rigid foam insulation to outside wall.

We continued nailing up the foam insulation.

We also applied that pink tape to the joints, which prevents a lot of air infiltration. We are relying on the foam to act as an air and water barrier, which is the function of house wrap materials.

For the foam to work properly and prevent drafts, it needs to be sealed tightly at the joints and where it meets windows, trim, etc.


We caulked the gap where the foam met the corner trim boards.

Any spots where the foam gets damaged (like when you miss the nail head) needs to be caulked or patched with tape. No, not duct tape.


We applied foam all the way to the peak of the gable, even though above that window awning the foam is useless, since the attic lies behind that part of the wall.

But we needed to put something on the wall to maintain that 1/2" thickness all the way to the top. Using the foam was the easiest method.


More Felt:

After the foam had been installed we applied another layer of 15 pound felt. Many people would say this felt is not needed, but I have my reasons for using a second layer.

It's cheap, it's fairly easy, I can't see any harm, and it will form a very thin layer of slightly breathable material that might help errant water dissipate. Maybe. And there is another reason I'll explain in the article about installing fiber-cement siding.


Why Did We Use Half-Inch Thick Foam Insulation?

It was a decision between 1/2" and 3/4" foam. Anything thicker would have made the new siding protrude too far from the existing window and corner trim. I would prefer the thicker insulation, but the available length of nails for the fiber-cement siding limited how much space we could put between the  wall sheathing and the siding.

Hardiplank® siding can be installed with roofing nails and a coil roofing nail gun. This greatly speeds up the siding installation process. But the longest roofing nails our nail gun could accept was 1-3/4".

The siding is a little more than 1/4" thick, I want the nails to penetrate one inch into the wood. That leaves an additional 1/2" of nail length... so 1/2" foam is the thickest we could use.


Read the next article in the series: Building Traditional Water Table Trim

Or skip it and read: Installing Fiber Cement Siding


HammerZone's Recommended Remodeling Tools



Tools Used:

  • Cordless Impact Driver
  • Basic Carpentry Tools
  • Deck Screw Gun With Collated Screw Feeder
  • Drywall T-Square
  • Ladders, Heavy-Duty

Materials Used:

  • Deck Screws, Collated
  • Polyisocyanurate Foam Insulation, ½" Thick
  • Cap Nails
  • 15# Roofing Felt (Tar Paper)

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Written March 25, 2005